A laser may be used as an optical transmitter that transmits light at a given wavelength. The power (i.e., amplitude) of the laser light is modulated by corresponding modulation of the power used to drive the laser. For example, the optical transmitter may be directly modulated to carry a wide-band RF signal. In a directly-modulated electrically pumped semiconductor laser, the electrical current that drives or pumps the laser is modulated.
The use of a laser modulated to carry a wide-band RF signal results in distortion due to the non-linear nature of the laser device. Distortion includes even-order distortion and odd-order distortion. Even-order distortion includes composite second order (CSO) distortion products, i.e. distortion products of the type 2f1, 2f2, f2−f1, and f2+f1. In particular, CSO is a second-order distortion that combines signals at frequencies A and B, as A±B. Odd-order distortion includes composite triple beat (CTB) distortion. CTB (also known as C/CTB) is a third-order distortion product that combines signals at frequencies A, B, and C as A+B−C.
Several techniques have been proposed or employed to compensate for distortion by injecting distortion of equal magnitude but opposite phase to the distortion produced by the laser device. For example, a predistortion circuit may be employed to predistort the RF signal being applied to modulate the laser. One such predistortion circuit includes split signal paths—a main or primary signal path and a secondary signal path. A small sample of the RF input is tapped off the main signal path and a distortion generator in the secondary signal path generates distortion (i.e., predistortion). The predistortion is then recombined with the RF signal on the main signal path such that the predistortion is of equal magnitude but opposite sign to the laser-induced distortion.
Such existing predistortion circuits may not be effective to compensate for distortion over a wide range of frequencies and particularly at higher frequencies, for example, in broadband applications such as forward-path CATV applications. Non-linear laser devices may have amplitude or magnitude and phase distortion characteristics dependent on the frequency of the modulating signal. Such frequency dependent nonlinearities are sometimes referred to as frequency tilt. Existing predistortion circuits may require other components, in addition to the distortion generator, to adjust the magnitude, frequency tilt and phase of the predistortion to match the laser-induced distortion.